The Pakistani Taliban is claiming responsibility for a deadly attack inside a military-run school in Peshawar, in northwestern Pakistan, on Tuesday, that has left as many as 145 dead, more than 100 of them students. The BBC has described the attack as the deadliest massacre ever carried out by the Taliban in Pakistan.

Gunmen entered Army Public School and Degree College by scaling the walls of the campus’ main building. The attackers held students hostage for more than eight hours, as they moved systematically from classroom to classroom firing at children. Reuters quoted a local hospital as saying that the dead and injured were aged between 10 and 20 years old.

Six gunmen were reportedly killed in the gunfire. A spokesperson for the terrorist group says the massacre was a retaliation against earlier Pakistani military activities against militants in North Waziristan.

“We selected the army’s school for the attack because the government is targeting our families and females,” Taliban spokesman Muhammad Umar Khorasani said, according to Reuters. “We want them to feel the pain.”

Prime Minister Nawaz Sharif, who has traveled to Peshawar, has called for three days of national mourning.